As the semiconductor devices keeps scaling down in size, three-dimensional multi-gate structures, such as the fin-type field effect transistors (FinFETs), have been developed to replace planar Complementary Metal Oxide Semiconductor (CMOS) devices. A structural feature of the FinFET is the silicon-based fin that extends upright from the surface of the substrate, and the gate wrapping around the conducting channel that is formed by the fin further provides a better electrical control over the channel.
Currently, to achieve n-type FinFETs and p-type FinFETs with low and symmetrical threshold voltages (Vth), different work function metals are used in metal gate fabrication of n-type FinFETs and p-type FinFETs. During the current metal gate etch back process, yield rate and reliability of the FinFETs may deteriorate.